It is common practice for a portable electronic device, for example a mobile telephone or a laptop computer, to be powered by a rechargeable chemical battery. Generally speaking, such a battery is releasably connected to the body of a portable device.
The use of a battery for supplying power to a portable electronic device is not ideal because the energy storage capacity of a chemical battery is limited. As such, it is necessary for the chemical battery to be recharged at regular intervals.
In order to provide a means for recharging the battery, the portable device is normally supplied with a charger unit to allow electrical energy to flow from a mains power supply to the rechargeable battery. The charger unit conventionally comprises an electrical plug for connecting to a mains power supply socket and an electrical cable for connecting the electrical plug to the portable device.
This is disadvantageous because, if there is no convenient mains power supply socket, as is the case in most outdoor and public environments, the rechargeable battery will run out of power and the portable device will need to be switched off.
The use of such a charger unit is further disadvantageous in that it requires a physical connection between the portable device and a mains power supply socket. This severely restricts the movement of the portable device during charging, thereby negating the portability of the device.
Another type of charger unit makes use of the principle of conventional, short-range inductive coupling, which involves the transfer of energy from a primary inductor in a charger unit to a secondary inductor in the portable device. Such charger units are commonly used, for example, for charging rechargeable batteries in electric toothbrushes.
Chargers utilising this type of conventional inductive coupling are able to transfer power wirelessly and hence do not require a physical connection between the mains supply and the portable device. However, the maximum distance over which effective power transfer can be achieved is limited to distances of the same order of magnitude as the physical dimensions of the inductors. For portable electronic devices, the dimensions of the inductor are limited by the size of the portable electronic device. Accordingly, in general, at distances of anything greater than a few centimeters, the efficiency of energy transfer between primary and secondary inductors is too small for this type of power transfer to be viable.
Therefore, as with the electrical cable discussed above, power transfer using conventional inductive coupling requires the charger unit and the portable device to be in very close proximity, meaning that the movement of the portable device is severely restricted.
In addition to the above problems associated with recharging, the use of a chemical battery as a power supply presents a number of further disadvantages. For example, rechargeable chemical batteries have a limited lifespan and tend to experience a decrease in their maximum storage capacity as they get older. Furthermore, chemical batteries are relatively heavy, meaning that the inclusion of a chemical battery in a portable device generally adds a significant percentage to the device's overall weight. If the device's reliance on the chemical battery could be reduced, then it would be possible for portable electronic devices such as mobile telephones to become significantly lighter.